1. Field of the Invention
The present invention relates to a card-type wireless communication device mounted in an information terminal device such as a PC (Personal Computer), for constructing a wireless LAN.
2. Description of the Related Art
In recent years, data communication via a wireless LAN (Local Area Network) typified by, for example, standards of IEEE802.11 series is becoming common. In the wireless LAN, a wireless network using an electric wave is constructed, and data transmission/reception between PCs or a PC and a peripheral device such as a printer can be carried out.
For example, in IEEE802.11b, an electric wave of a 2.4 GHz band is used, and a wireless network having a data transfer rate of about 10 Mbps almost equal to that of a low speed wired LAN can be constructed in the range of a distance of about 10 m to 100 m. In the IEEE802.11b, wireless communication by a direct sequence spectrum spread (DS-SS) as one of spectrum spread systems is adopted to prevent mixing of noise or tapping.
As a communication mode, there is an ad hoc system in which data transmission/reception is directly performed between transmission and reception devices, or an infrastructure system in which an access point is provided and data transmission/reception is performed through the access point. In order to realize the wireless LAN, a wireless LAN card or an adapter is mounted in each PC and peripheral device, and an access point is installed as the need arises.
Besides, recently, a wireless LAN card based on IEEE802.11a is going into actual use. This wireless LAN system uses a 5 GHz band and adopts an OFDM modulation system (Orthogonal Frequency Division Multiplex modulation system), so that a maximum transmission rate of 54 Mbps is realized.
FIG. 4 shows a longitudinal section of a conventional wireless LAN card of PCMCIA standards. FIG. 5 is a plan view of a printed board assembly of the conventional wireless LAN card. As shown in FIG. 4, in a conventional wireless LAN card 100, a printed board assembly 120 is attached to a frame 101 made of synthetic resin, and an upper surface and a lower surface of the printed board assembly 120 are covered with a top cover 102 and a bottom cover 103 which are made of metal.
As shown in FIG. 5, the printed board assembly 120 is structured such that a connector 122, a baseband circuit part 123, an RF (radio frequency) circuit part (wireless circuit part) 124 and two antennas 125 are disposed on a printed board 121. The connector 122 for connection with a not-shown information terminal device is attached to one end side of the substantially rectangular printed board 121 in the longitudinal direction. The diversity antennas 125 are disposed at the other end side of the printed board 121 in the longitudinal direction. The baseband circuit part 123 is disposed at the side of the connector 122. The RF circuit part 124 is provided at the side of the antennas 125. The RF circuit part 124 is covered with a shield case 126. A leg part of the shield case 126 is connected to a ground pattern (not shown) formed on the printed board 121 by soldering.
The wireless LAN can be constructed by mounting the wireless LAN card 100 in a card slot of an information terminal device such as a PC. In the state where the wireless LAN card 100 is mounted in the card slot, the portion of the antennas 125 protrudes outward. Since the portion of the antennas 125 is covered with the frame 101 made of synthetic resin, sending or receiving of an electric wave can be performed through one of the antennas 125. The baseband circuit part 123 is provided at a position remote from the antennas 125 in order to prevent a digital noise or the like generated in the baseband circuit part 123 from being mixed into the portion of the antennas 125. Further, since the RF circuit part 124 is covered with the shield case 126, it is possible to prevent a RF signal from leaking out and to prevent an external noise or the like from being mixed into the RF circuit part 124.
However, it has been found that when the wireless LAN card 100 having the structure shown in FIGS. 4 and 5 is mounted in each of plural personal computers and peripheral devices to construct a wireless LAN, and throughput of data transmission (data transmission amount per unit time) is measured, there is a case where the throughput is lowered.